You can enter a dungeon below recommended level, and the jump to adept is quite gentle. You need to do personal development goals to unlock skills and advance classes, each one you master also gives a stat point and HP permanently (the node after the permanent is not permanent, it's class prestige), cursed classes have 2, the second one gives equip immunity for their set . Most of the best moves are unlocked very early in a class so you're more reliant on your strategies, equipment, masteries, and stats than class level.